1. Field of the Invention
This invention relates to coated optical waveguide fibers and, in particular, to coated optical waveguide fibers wherein the coating's index of refraction is lower than that of the fiber's cladding.
2. Description of the Prior Art
Conventional optical waveguide fibers comprise a central glass core which is surrounded by a glass cladding. To protect the core and cladding, one or more layers of a polymeric material, such as, a UV-curable acrylate resin, are applied to the cladding.
To function as a waveguide, the index of refraction of the core must be higher than the index of refraction of the cladding. Typically, the cladding is made of essentially pure silica having an index of refraction (n.sub.D) of approximately 1.458, and the core is a doped silica having a slightly higher index of refraction, e.g., 1.462 for a typical single mode fiber.
Although an optical waveguide fiber will function with a protective coating whose index of refraction is lower than the index of refraction of the cladding, a protective coating having a higher index of refraction is generally preferred in order to eliminate the problems which can be caused by the propagation of energy in the cladding rather than in the core, i.e., to eliminate the problems caused by cladding modes.
Specifically, among other problems, cladding modes can cause bandwidth problems in fibers of short length and modal interference and amplitude (power) fluctuation problems when a fiber is used as a pigtail or jumper cable. In addition, cladding modes make the measurement of fiber properties, in particular, attenuation, more difficult and potentially inaccurate. Specifically, since fiber attenuation is normally determined by comparing the amount of light transmitted through the full length of a fiber with the amount of light transmitted through the first two meters of the fiber, a higher attenuation, which does not correspond to the true properties of the fiber, will be calculated if cladding modes are able to propagate through the first two meters of the fiber.
To deal with the cladding mode problem in measuring attenuation, existing techniques have employed complicated mechanisms for stripping cladding modes from fibers. For example, glycerine mode strippers have been placed at the launch and detection ends of the two meter fiber.
More generally, the cladding mode problem has been dealt with by using protective coatings whose indices of refraction are above that of the fiber's cladding. For example, fibers have been coated with a low modulus primary buffer having an index of refraction greater than 1.46 and with one or two layers on top of the primary buffer of higher modulus secondary buffers which provide strength and abrasion resistance. Alternatively, a single coating having an index of refraction greater than 1.46 has been used. In both cases, since the index of refraction of the coating in contact with the cladding is greater than the index of refraction of the cladding, total internal reflection of light at the cladding-to-coating interface will not occur. Accordingly, cladding modes are rapidly stripped out of the optical-quality cladding into the non-optical-quality coating where they are absorbed.
Unfortunately, reasonably-priced coatings which have a high index of refraction are in general lacking in other properties, such as, elastic modulus, coefficient of thermal expansion, moisture sensitivity, mechanical strength, and thermal stability. Looked at another way, the most desirable materials for use as coatings, i.e., materials which have one or more of the foregoing properties, typically have indices of refraction below that of silica.
For example, many silicone rubbers, such as, RTV (room-temperature vulcanizing) silicone rubbers, have a low elastic modulus over a wide temperature range, a low moisture sensitivity, and a high thermal stability up to 200.degree. C. Accordingly, these materials would make excellent coatings for optical waveguide fibers. Unfortunately, RTV silicone rubbers have a refractive index of approximately 1.42, well below the index of refraction of silica, and thus will not strip cladding modes from conventional single mode or multi-mode fibers.
In an attempt to overcome this problem, some manufacturers of optical waveguide fibers have applied a coating of a specially formulated silicone rubber, having an index of refraction of approximately 1.5, to the fiber's cladding, and have then coated this coating with a standard RTV silicone rubber. To try to offset the high cost of the high index silicone rubber, extremely thin coatings, e.g., on the order of 10 to 15 microns, have been used. Obviously, this approach makes the coating process more expensive, complex, and difficult to control.
Optical waveguide fibers having glass claddings which include an outer layer having a coefficient of expansion which is greater than that of the rest of the cladding have been proposed. See Kao et al., U.S. Pat. No. 4,243,298, and Schneider et al., U.S. Pat. No. 4,184,860. The Schneider et al. patent further states that the outer layer can be selected to have an index of refraction which is greater than the remainder of the cladding. Neither of these references, however, disclose or suggest using a high index of refraction layer so as to be able to directly coat the cladding of an optical waveguide fiber with a low index of refraction, high performance polymeric coating.